11.3.1 Treaties
Of these four sources, “international conventions,” also known as “treaties,” and
international custom (or customary law) are the most important ones. The central
role of treaties follows from a basic principle of traditional international law,
namely that a state is bound only by the rules of international law to which it has
specifically consented.
Voluntarism This principle reflects voluntarism, the idea that a state can only be
bound by an obligation after it has given its consent. Voluntarism follows from the
basic rule of state sovereignty. Since states represent the highest authority in the
international legal system, they are not required to accept any obligations they don’t
agree with. A state may express its consent, for example, by becoming a party to a
treaty in which the obligation in question is included.
The way in which treaties can be concluded is regulated by the1969 Vienna Convention on
the Law of Treaties.
A state therefore is entirely free to join or not to join international legal
instruments such as the UN Convention against Climate Change. A state may feel
politically pressured to join, but it is under no legal obligation to do so.
11.3.2 Ius Cogens
Peremptory NormsThe traditional voluntaristic approach is being undermined
by the emergence ofius cogensor peremptory norms of international law. Rules of
ius cogensor peremptory norms are the highest in rank, and they override any
contrary international obligations of a state. Examples of rules ofius cogensare the
prohibition of genocide and the prohibition of aggression. Article 53 of the 1969
Vienna Convention on the Law of Treatiesprovides the following description:
A treaty is void if, at the time of its conclusion, it conflicts with a peremptory norm of
general international law. For the purposes of the present Convention, a peremptory norm
of general international law is a norm accepted and recognized by the international
community of States as a whole as a norm from which no derogation is permitted and
which can be modified only by a subsequent norm of general international law having the
same character.
It follows that a treaty in which two states agree to commit aggression against
another state is null and void because it is incompatible with the rule ofius cogens
prohibiting such conduct.
Such a hierarchical structure of norms makes international law more comparable
to domestic legal systems in which laws override regulations and constitutional
provisions override ordinary laws.
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